I thought I'd put up a post here of something I learned with my last set of bettas and that I've researched more thoroughly lately. Most people don't seem to much pay attention to the kH of water, basically the carbonate hardness. It seems that if your KH is too low, your pH can crash, thereby causing your fish (and anything else in your tank) some health difficulties. I had some beautiful halfmoons last year, and although i kept them spotless, they both developed fin rot. They weren't in the same tank and I didn't share items without washing them first. I fought with this illness, treating the finrot, for months, and eventually they lost almost all of their beautiful fins and they never grew back. The breeder tipped me off to the KH difficulties and suggested i add 1/4 tsp baking soda to each gallon of water. I started doing that and the "fin rot" disappeared. Upon further research with my new betta and my new saltwater tank, it seems that KH, something we don't often think of, can really affect the bettas and cause illness because of fluctuating pH levels, which we all know aren't good for our little friends.
So if you notice your betta constantly getting sick, you may want to consider the KH content. Baking soda acts as a great buffer and won't adversely affect your little friend if used properly. It's really inexpensive and can very much help extend your betta's life.

I'm glad you all find it helpful. :) I discussed it with my tank store today. They still recommend using the tap water (i'm in Chicago) for my guy so I guess i'll watch out and see if it's going well. When I was originally doing it, i was doing it as a preventive because it was the only thing keeping them from getting the fin rot back. I think there are test kits of kh, but I don't have one. I did test the calcium content of the tap using the saltwater kit i have and it showed pretty much zero. i'm finding that what's most important is consistency. remember that when we put the water into the tank, the ph will adjust depending on other factors such as substrate and old water. So maybe because I was doing the large water changes, it was causing pH swings. Perhaps adding the baking soda just kept it buffered, basically not allowing it to swing.
let's hope my new little guy stay well with this chicago water!

This is really interesting, I've never tested my KH just my pH and I did notice that it tends to fluctuate from around 7-7.8 and my betta has had bad fin rot for months now. I've been trying everything and have been considering using prescription amoxicillin to help him. I do my next water change tomorrow and I think I'll try this out and see how it goes. Is it necessary to add baking soda between water changes or just every time you change the water? Also is this safe to use with live plants?

No, not between, just with the new water changes. As long as it goes in with the new water, that should help stabilize it. I've never kept live plants, so I can't say for sure, but I don't think it should be harmful. This website says it's actually beneficial to your plants.http://www.yamatogreen.com/co2.htm
Good luck! Let us know how it goes, please!